In hydraulic drive traveling systems for construction vehicles such as wheel loaders or power excavators, a hydraulic pump of variable displacement type is used as a drive source for traveling, and the capacity of the hydraulic motor is varied at two speeds dependent on changes in the traveling load produced due to a slope of the road surface, the nature of the ground, etc. so that two-speed control is performed while traveling. In order to eliminate trouble or intricacy imposed on an operator for varying the capacity, one of such hydraulic drive traveling systems as disclosed in JP.U. 63-54521 is designed to vary the capacity automatically.
More specifically, the disclosed conventional hydraulic drive traveling system comprises an engine, a hydraulic pump driven by the engine, a traveling hydraulic motor of variable displacement type driven by the hydraulic pump, a pressure sensor for detecting a delivery pressure of the hydraulic pump, and motor control means for comparing the pump delivery pressure detected by the pressure sensor with a setting value, and changing a swash plate tilting angle of the hydraulic motor between a large tilting (large capacity) and a small tilting (small capacity) from one to the other dependent on the compared result.
During operation under the high traveling load, e.g., while ascending a sloped road, the delivery pressure of the hydraulic pump is increased and the pressure sensor detects the increased pump delivery pressure. When the detected pump delivery pressure exceeds a first setting value, the motor control means shifts the swash plate tilting angle of the hydraulic motor toward the large tilting side. The hydraulic motor is thereby set to a low-speed, large-torque mode for providing traction force enough to ascend the sloped road. When coming into operation under the low traveling load, e.g., traveling on a flat road, the delivery pressure of the hydraulic pump is reduced and the pressure sensor detects the reduced pump delivery pressure. If the detected pump delivery pressure falls below a second setting value, the motor control means shifts the swash plate tilting angle of the hydraulic motor toward the small tilting side. The hydraulic motor is thereby set to a high-speed, low-torque mode for permitting the traveling at a high speed.
Thus, in the conventional system, the delivery pressure of the hydraulic pump is detected and the detected pump delivery pressure is compared with the first and second setting values. Dependent on a result of the comparison, the capacity of the traveling hydraulic motor is carried out to automatically perform two-speed control of the traveling. In the conventional system, however, when traveling on a flat road under the low traveling load with fine control operation, the delivery pressure of the hydraulic pump is not increased so that the hydraulic motor is always shifted toward the small tilting side and hence controlled in the high-speed, low-torque mode. This creates a problem of difficulty in achieving a wide speed range from low speeds to high speeds in a usual traveling condition.
An object of the present invention is to provide a hydraulic drive traveling system which can take a wide speed range from low speeds to high speeds even in a condition under the small traveling load.